Updating UI components from an async callback (dispatch_queue)

Question

how can i update GUI elements with values from a queue? if i use async queue construct, textlable don't get updated. Here is a code example i use:

- (IBAction)dbSizeButton:(id)sender {
    dispatch_queue_t getDbSize = dispatch_queue_create("getDbSize", NULL);
    dispatch_async(getDbSize, ^(void)
    {
        [_dbsizeLable setText:[dbmanager getDbSize]]; 
    });

   dispatch_release(getDbSize);
}

Thank you.

Did you try performSelectorOnMainThread:withObject:waitUntilDone:? — Hejazi
– Hejazi, Commented Jan 3, 2012 at 20:27

ferostar · Accepted Answer · 2012-01-03 20:30:59Z

9

As @MarkGranoff said, all UI needs to be handled on the main thread. You could do it with performSelectorOnMainThread, but with GCD it would be something like this:

- (IBAction)dbSizeButton:(id)sender {

    dispatch_queue_t getDbSize = dispatch_queue_create("getDbSize", NULL);
    dispatch_queue_t main = dispatch_get_main_queue();
    dispatch_async(getDbSize, ^(void)
    {
        dispatch_async(main, ^{ 
            [_dbsizeLable setText:[dbmanager getDbSize]];
        });
    });

    // release
}

answered Jan 3, 2012 at 20:30

ferostar

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3 Comments

Catfish_Man Over a year ago

Except you'd probably want the -getDbSize occurring on the private queue, not the main queue...

JohnK Over a year ago

@Catfish_Man: But isn't getDbSize the private queue?

Catfish_Man Over a year ago

It's not happening on the getDbSize queue. It's happening in a dispatch_async'd block running on the main queue. You need it inside the first dispatch_async, but outside the second.

Mark Granoff · Accepted Answer · 2012-01-03 20:29:52Z

2

Any UI update must be performed on the main thread. So your code would need to modified to use the main dispatch queue, not a queue of your own creation. Or, any of the performSelectorOnMainThread methods would work as well. (But GCD is the way to go, these days!)

answered Jan 3, 2012 at 20:29

Mark Granoff

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